Nickel powder and a process for producing it

ABSTRACT

1. The method of producing nickel powder which comprises reacting oxalic acid with aqueous nickel nitrate to produce nickel oxalate, heating the nickel oxalate in an atmosphere of carbon dioxide at a temperature in the range of 450*C. to 850*C. in the presence of a small amount of a particle grain growth inhibitor to produce a substantially pure nickel powder of uniform particle size and thereafter heating the powder in a mildly oxidizing atmosphere at an elevated temperature of less than 100*C. to render the powder non-pyrophoric.

United States Patent [1 1 Pall 1 1 Dec. 2, 1975 NICKEL POWDER AND A PROCESS FOR PRODUCING IT [75] Inventor: David B. Pall, Forest Hills, NY.

[73] Assignee: The United States of America as represented by the United States Energy Research and Development Administration, Washington, DC.

[22] Filed: Apr. 26, 1945 [21] Appl. No.: 590,427

[52] US. Cl. 75/.5 AA; 148/126 [51] Int. Cl. B22F 9/00 [58] Field of Search 75/82, 170 A, .5, .5 A; 148/126 [56] References Cited UNITED STATES PATENTS 455,228 6/1891 Mond 75/82 2,254,976 9/1941 Powell 75/.5

FOREIGN PATENTS OR APPLICATIONS United Kingdom 75/34 419,953 ll/l934 OTHER PUBLICATIONS Hirschkowitsch, Uber die Zersetzung der Oxalute, ln Zeitschrift fur Anorganische und Allgemeine Chemie. Vol. 115, pp. 159-167,1921.

Primary Examiner-Benjamin R. Padgett Attorney, Agent, or Firm-Dean E. Carlson; Leonard Belkin EXEMPLARY CLAIM 3 Claims, 4 Drawing Figures US. Pamnt Dec. 2, 1975 Sheet 1 of2 3,923,496

N-36 SERIES '5 850 L) 51750 8 650 E 55h 9 a H 450 I q DIAMETER MIGRONS Fig. l

GROWTH ENHIBITING MATERIAL AFJPROXIMATE ARBITARY CONTENT, "/a

UNITS CALCINATIOM TEMPERATURE To REACH 1.05 30 Fig. 2

INVENTOR DAVID B. FALL ATTO RNEY APPROXIMAIE 9 .025 CONTENT,

INVENTOR .DAVID B. PALL Sheet 2 of 2 Fig. 3

IV-3| SERIES TO 493%, wcnons Dec. 2, 1975 PARTICLE DIAMETER ON CALCINATION O O O O 0 w w a w U. S. s atent GR W H l N HI 8 ITIN'G MATERIAL IN ARBITR ARY U NITS DAMETERS MIC-RUNS NICKEL POWDER AND A PROCESS FOR PRODUCING IT This invention relates to the preparation of nickel powder in finely divided form by a process which can be controlled to produce selected sizes, and to the products produced by the process.

Nickel powders have many uses in industry and in the laboratory, among which is the production of those porous diaphragms which are used for separating the components ofa gas. The requirements of those porous diaphragms are severe; they must be ductile, strong, and capable of being exposed to corrosive influences without disintegration or loss of property. Inasmuch as this particular use of nickel powder is of some importance, the invention will be described with repeated reference to the production of nickel powders for use in such porous diaphragms, but it is to be understood that the powders which are produced by this process are equal or superior to the prior art powders in various other uses to which they may be put, and that this particular reference is in no sense a limitation.

Technicians and research chemists require a nickel powder the particles of which approximate a single particle size, but heretofore a variety of circumstances made the production of uniform powders a matter of great difficulty.

Not only must the nickel approximate a single particle size but it must be substantially free from hard aggregates if it is to be satisfactorily used in porous diaphragm construction. Most nickel powders show the phenomenon of aggregation. An aggregate is a group of particles held together by an inter-crystalline growth or by some cementing material. Groups held together by surface forces only are not considered to be aggregates under the terminology herein used, largely because such groups are easily broken down to single particles whereas the hard aggregates are not. The aggregates formed during the preparation of nickel powders by some processes are so integrated and so hard that they deleteriously affect the characteristics of the diaphragm in which they are included, Furthermore, some of the prior art processes of producing nickel powders do not produce products which sufficiently approximate a desired particle size. Other processes were not sufficiently flexible in operation when it was desired to vary the particle size produced.

It is an object of the invention to produce finely divided nickel in selected particle size substantially free from hard aggregates by a process which is adaptable to plant use. A further object of the invention is to produce metallic nickel in powder form by pyrolysis. Yet 4 another object of the invention is to produce nickel powder in selected particle sizes by a process involving heat treatment.

The objects of the invention are accomplished, generally speaking, by precipitating nickel from a nickel salt bath in a form that may be decomposed by heat, decomposing the precipitated nickel compound by pyrolysis, and heat treating the product under controlled conditions, the nature of which are more fully described hereinafter.

The drawings attached to the specification are all graphs and will be described in their proper place.

Nickel powders can be made by this process in a range from about 0.5 microns to as high a diameter as is required. For instance, 40 micron powder has been prepared and does not represent the upper limit of the process. Within reasonable limits, the distribution will vary from intermediate to very wide; and in some cases an extremely narrow range of particle sizes can be obtained by adding appropriate growth inhibitors to the minimum diameter which can be produced, lowering it to about 0.1 microns. Even at this small diameter, tests have shown the nickel powders to be equal to, or superior to, other powders which have been tested for the manufacture of porous diaphragms.

Examples will be included from place to place in the specification to illustrate particular portions of the process as the description proceeds.

The thermal decomposition of nickel oxalate follows the simple equation:

Nic o 2 H O Nic o, 2 H O NiC Q, Ni 2 CO Pyrolysis has been successfully carried out in glass, quartz, and steel reaction chambers. Stainless steel reaction chambers may be used but standard steel chambers are satisfactory in most instances. The apparatus is so constructed that the pyrolysis can be carried out in an atmosphere of an inert gas such as carbon dioxide or nitrogen. Using the standard steel pyrolysis equipment, carbon dioxide or other inert gases may be run through the reaction chamber at considerable velocity. The presence of water vapor does not appear to be harmful. A hydrogen atmosphere has been used as a finishing step, but particle growth seems to occur in its presence at lower temperatures, although not to a harmful extent. Control of sizes in the process is carried out by the tinting strength method that is described in a co-pending application and which is, so far as is known, the only fast method of determining particle sizes of finely divided metal powders.

EXAMPLE 1 A gas fired furnace of the type used for carburizing steel parts was adapted for this purpose. The muffle is a steel shell 20 inches" in diameter and 36 inches long, which is rotated at 23 rpm. End to end mixing is achieved by a rocking motion, about once every 2 minutes, which tips the muffle to an angle of about 7 in either direction. A shielded thermocouple comes in through a rotary, gastight joint and the rear hollow shaft of the muffle, and is bent at right angles so that the couple junction is always covered by powder. A gas inlet is also arranged through the rear shaft. The muffle is filled and cleaned through a 12% inch front opening. The closure is a close fitting flanged cylinder, which contains a 5 inch air space for insulation, and is gasketed with asbestos cord. This closure is provided with a 1 inch center opening, through which a sampling device may be inserted, or out of which a larger sample may be run into an attached flask by tipping the whole calciner forward at a sufficient angle.

A set of six gas burners is located under the muffle, and the whole surrounded by an insulated shell which is mounted on trunnions for filling and emptying.

In this apparatus, 18-20 pounds of metal powder can be prepared from the appropriate quantity of nickel oxalate hydrate in about four hours. As in the case of small scale operation, control is by tinting strength. Results appear to be identical in both types of calcination, provided the same ultimate diameter is reached.

Nickel oxalate filterpress cake contains only about 7% of water, and for this reason the problem of drying is not a serious one. This is particularly true because the product may be dried at temperatures as high as 200C without harm.

At 200C and higher, loss of water of crystallization begins to occur. Complete removal of this water in an oxidizing atmosphere is not recommended, unless thorough mixing can be simultaneously achieved (as in a rotary drier). When dried in stationary trays, decomposition to metal is liable to occur at temperatures as low as 225C; on the other hand the dehydration is quite slow at 225C. It is advisable to dehydrate and decompose the nickel oxalate in the same apparatus and the same operation when operating on a commercial scale.

After the pyrolysis, the nickel powder may be slowly aerated for a considerable period of time, for instance overnight, to render its non-pyrophoric.

EXAMPLE 2 Preparation of a 0.4 micron nickel powder 77.1 pounds of a special reagent grade nickel nitrate (Ni(NO 6 H O) are dissolved; the solution is clarified through a small filter press, pumped to a 75 gallon glass-lined tank, and made up to a 2 molar solution by the addition of H and steam. Then 0.035% magnesium (figured on nickel basis) as 4.1 grams MgO which had been completely dissolved in HNO is added. 36.8 pounds of technical oxalic acid (14 C 0 2H O) are dissolved in hot water, and made up to a 2 molar solution in a gallon stoneware vessel. The nickel nitrate solution is then heated to 97C, and the oxalic acid solution is heated to 85C. The oxalic acid solution is added to the nickel solution uniformly over a ten minute period; the strike is maintained at 97C for 20 minutes. The precipitate is decanted with interim washing three times, and the solid nickel oxalate is collected in a wash press, washed and dried at 110C for approximately 18 hours.

All the nickel oxalate (NiC O 2H O) approximately 44 pounds is placed in a large rotary calciner externally heated by gas. The powder is then heated at a uniform rate in a C0 atmosphere, until the temperature reaches 450C at the rate of approximately 100C per hour. The calciner is rotated at 23 rpm and rocked forward and backward mixing while the temperature rises from 390C to 450C. The powder is then cooled to less than 100C, and aerated overnight with a slow stream of air to render it non-pyrophoric. Finally, it is removed from the calciner, and passed through a mechanical disintegrator.

This powder has a mean crystal diameter averaged by volume of 0.4 microns, as determined by the tinting strength method. An estimate of the size by means of the electron microscope gave the same result. An analysis for magnesium showed only 0.008% magnesium (figured on nickel basis) which shows that only approximately one-fourth of the magnesium was carried down with the nickel oxalate precipitate.

EXAMPLE 3 When the previous example if repeated, carrying the temperature to 500C, the diameter of the powder is 0.5 micron, by the tinting strength method.

EXAMPLE 4 Preparation of a 0.1 micron nickel powder 154.2 pounds ofa special reagent grade nickel nitrate (Ni(NO .6H O) are dissolved in water and filtered through a clarification press. The solution is pumped to a stainless steel tank, and made up to a 2 molar solution by the addition of water and steam. Then 0.65% magnesium (figured on nickel basis) or 149.2 grams MgO which was completely dissolved in HNO are added. 73.6 pounds of technical oxalic acid (H C O .2H O) are dissolved in hot water and made up to a 2 molar solution in a 40 gallon crock. The nickel nitrate solution is then heated to 97C, and the oxalic acid solution is heated to C. The oxalic acid is added uniformly over an 18 minute period, and the strike is digested at 97C for 20 minutes. Washing by decantation is repeated three times; the nickel oxalate is filtered in a wash press, washed and dried at 110C for approximately 18 hours.

The nickel oxalate (NiC- O .2H O) approximately 50 pounds is then placed in a large rotary calciner, heated by gas. The powder is then heated at a uniform rate of C per hour in a C0 atmosphere, until the temperature of the powder reaches 460C, (the furnace is rotated at 23 rpm and rocked from 390C 460C to get end to end mixing). The powder is cooled to less than 100C, then aerated overnight with a slow stream of air to render it non-pyrophoric, removed from the calciner, and micropulverized.

This powder has a mean crystal diameter averaged by volume of 0.1 of a micron, as determined by the electron microscope. An analysis for magnesium showed only 0.11% magnesium (figured on nickel basis) which shows that only approximately one-sixth of the magnesium was precipitated.

The particle size of fine nickel powders increases very rapidly if they are heated above about 400C. The increase in particle size is affected by both the time and the temperature, maximum size being reached after about 15 to 30 minutes. Because of this fact, the preparation of nickel powders of selected size would be technically precluded except for the discovery of particle size inhibitors of the class consisting of magnesium, the alkali metals and the alkaline earth metals. When a member of this group is present during heating, even in very minute quantities, particle growth is markedly restrained. Substantially complete reduction of nickel oxalate to nickel powder is accomplished by heating pure nickel oxalate in a non-oxidizing chamber at a temperature around 450C. If no inhibitor is present the particle size of the product will be of the order of 1 micron, and as the heating is continued or as the temperature is raised, the particle size will increase with some rapidity. However, if a small amount of magnesium be present, a very much smaller particle size is obtained. The relationship of the various factors such as the percentage of inhibitor, and the temperature are shown in the accompanying drawings, and will be more particularly described in connection therewith.

FIG. 1 is a graph showing the temperature in degrees Centigrade plotted against the diamter of the nickel powders in microns, all experiments having been carried out for equal periods of time. Thus, it is shown that at 450C, a size of between 1 and 2 microns is produced. At about 550C the size is between 2 and 4 microns. At 650C the size is about 9 microns. At 750C it has reached about 16 microns.

In FIG. 2 the growth inhibiting material, in this case magnesium, is plotted against the temperature required to reach a particle size of 1.5 microns, the temperature being given in degrees Centigrade. Thus it is observed that this average diameter is attained at about 450C with an extremely small percentage of magnesium, based on the weight of the nickel, and that it is attained at a temperature just over 550C if 0.025% of magnesium be included. A particular advantage of this phase of the invention is that higher temperatures can be employed than were advantageous prior to the discovery.

In FIG. 3 the magnesium content is plotted against the particle diameter produced at a temperature of 493C. This chart shows that with a magnesium content of .05% heating to 493C produced nickel powder having a particle size of about 0.6 microns; that with a content of about 0.025% magnesium, a particle size of 0.7 microns was obtained; and that as the magnesium content was reduced, the temperature being maintained at the same level, the size of the particles increased, approaching 2 microns as the magnesium content approached 0.001%.

FIG. 4 shows the effect of temperature on particle size in the presence of a relatively high percentage of magnesium. FIG. 1 represents the size-temperature relationship of a sample having a magnesium percentage of about 0.001. FIG. 4 represents the heat treatment of nickel powders containing about 0.3% magnesium. By comparing FIG. 1 and FIG. 4 an appreciation will be gained of the close control of particle size which can be secured by the process of this invention.

When the control of particle size is to be effected by the alkali metals or alkaline earth metals, other percentages will be required to obtain optimum results, depending upon the technical efficacy of the particular metal.

It is very important that the inhibitor be uniformly distributed throughout the mass prior to pyrolysis. This distribution can best be obtained by dissolving the appropriate percentage of magnesium with the nickel while it is in solution, so that nickel and inhibitor will be co-precipitated.

The process herein described of controlling the particle size of nickel powders has been set forth largely in connection with powders produced by the preferred oxalate process, but the invention is not so limited. It is in fact applicable to nickel powders made from such intermediates as nickel oxide, nickel carbonate, nickel chloride, and nickel carbonyl. The several intermediates produce different particle sizes under similar conditions, so that it is possible by means of this invention and the use of a selected intermediate to produce nickel powder approximating any selected average particle size, and to control the formation of aggregates so that they do not constitute a serious problem.

An advantage of the invention is that an adequate control of nickel powder particle sizes is attained by the addition of small amounts of an inhibitor prior to heat treatment. Another advantage of the invention is that nickel powders in selected sizes are produced by a process whose controls are adapted to ordinary plant manufacture. Another advantage of the invention is in the coordination of the techniques of precipitation, pyrolysis and heat treatment in attaining the objects of the invention. Another advantage of the invention is in the use of heat treatment to produce nickel powder in a size approximating a selected average. Another advantage of the invention is that the pyrolysis technique may be employed for the reduction of many nickel salts and is not limited to those produced by the precipitation technique. Another particular object of the invention is to make use of inhibitors, combined with other steps of the process, to yield results which would otherwise be incapable of accomplishment in a satisfactory degree. Another advantage lies in the fact that successive batches of raw materials for the manufacture of metal powders vary in their content of minor impurities, and therefore of growth inhibitors, but by purposely adding a constant preponderant quantity of growth inhibitor, the metal powders obtained will always show reproducible behavior on heating.

The use of particle growth inhibitors is not limited to the metal produced by the pyrolysis of nickel oxalate alone, but may be used to control particle growth during the heat treatment of metal powders obtained from other salts. A particular advantage of the invention springs from the use of an inert gas during pyrolysis followed by aeration to reduce pyrophoric tendencies of those powders which have them as a characteristic or as an attribute of particle size. Other advantages of the invention will be apparent to persons skilled in the art to which the invention applies.

1 claim:

1. The method of producing nickel powder which comprises reacting oxalic acid with aqueous nickel nitrate to produce nickel oxalate, heating the nickel oxalate in an atmosphere of carbon dioxide at a temperature in the range of 450C. to 850C. in the presence of a small amount of a particle grain growth inhibitor to produce a substantially pure nickel powder of uniform particle size and thereafter heating the powder in a mildly oxidizing atmosphere at an elevated temperature of less than C. to render the powder nonpyrophoric.

2. The process of producing nickel powder which comprises adding oxalic acid to aqueous nickel nitrate containing dissolved therein a small amount of a particle grain growth inhibitor selected from the group consisting of magnesium, the alkaline-earth and the alkali metals to coprecipitate nickel oxalate and an oxalate of said grain growth inhibitor, heating said coprecipitate in an inert atmosphere at a temperature in the range of 450C. to 850C. to produce a substantially pure nickel powder of uniform particle size and thereafter heating said nickel powder at an elevated temperature less than 100C. in a mildly oxidizing atmosphere to render the nickel powder non-pyrophoric.

3. The method of producing nickel powder which comprises adding oxalic acid to an aqueous solution of nickel nitrate containing dissolved therein a small quantity of magnesium nitrate to produce a coprecipitate of nickel and magnesium oxalates containing 0.02 to 0.3% magnesium based on the nickel weight, heating said coprecipitate at a temperature of the order of 450C. to produce a substantially pure nickel powder of uniform particle size and immediately thereafter heating said nickel powder in a mildly oxidizing atmosphere at an elevated temperature less than 100C. to

render the nickel powder non-pyrophoric. 

1. THE METHOD OF PRODUCING NICKEL POWDER WHICH COMPRISES REACTING OXALIC ACID WITH AQUEOUS NICKEL NITRATE TO PRODUCE NICKEL OXALATE, HEATING THE NICKEL OXALATE IN AN ATMOSPHERE OF CARBON DIOXIDE AT A TEMPERATURE IN THE RANGE OF 450*C. TO 850*C. IN THE PRESENCE OF A SMALL AMOUNT OF A PARTICLE GRAIN GROWTH INHIBITOR TO PRODUCE A SUBSTANTIALLY PURE NICKEL POWDER OF UNIFORM PARTICLE SIZE AND THEREAFTER HEATING THE POWDER IN A MILDLY OXIDIZING ATMOSPHERE AT AN ELEVATED TEMPERATURE OF LESS THAN 100*C. TO RENDER THE POWDER NON-PYROPHORIC.
 2. The process of producing nickel powder which comprises adding oxalic acid to aqueous nickel nitrate containing dissolved therein a small amount of a particle grain growth inhibitor selected from the group consisting of magnesium, the alkaline-earth and the alkali metals to coprecipitate nickel oxalate and an oxalate of said grain growth inhibitor, heating said coprecipitate in an inert atmosphere at a temperature in the range of 450*C. to 850*C. to produce a substantially pure nickel powder of uniform particle size and thereafter heating said nickel powder at an elevated temperature less than 100*C. in a mildly oxidizing atmosphere to render the nickel powder non-pyrophoric.
 3. The method of producing nickel powder which comprises adding oxalic acid to an aqueous solution of nickel nitrate containing dissolved therein a small quantity of magnesium nitrate to produce a coprecipitate of nickel and magnesium oxalates containing 0.02 to 0.3% magnesium based on the nickel weight, heating said coprecipitate at a temperature of the order of 450*C. to produce a substantially pure nickel powder of uniform particle size and immediately thereafter heating said nickel powder in a mildly oxidizing atmosphere at an elevated temperature less than 100*C. to render the nickel powder non-pyrophoric. 